How to load and display an image in the image editor using python?
If the image editor is used in one of the visible areas of the current workspace, you can iterate through bpy.context.screen.areas to find the one with type IMAGE_EDITOR. The image that is shown in the editor can be set through the space of the area.

An image can be loaded through bpy.data.images.load().

import bpy


img = bpy.data.images["Render Result"]
# img = bpy.data.images.load(filepath)

for area in bpy.context.screen.areas:
    if area.type == 'IMAGE_EDITOR':
        area.spaces.active.image = img